Not content with eating into Microsoft's share of the browser market with Firefox, the Mozilla Foundation has released an open-source email client to rival Outlook.

Mozilla Thunderbird 1.0 is available from Tuesday on Windows, Mac OS X and Linux. It includes a built in spam filter, an RSS reader and improved search and sorting capabilities; users can save the results of a search in a folder and run the search again at a later date, or can colour-code messages to help sorting.

The open-source email client also has some mobility features, including offline functionality and Palm address book synchronisation. To ease migration for users of other email systems, the application automatically imports account settings and addresses from Microsoft Outlook, Microsoft Outlook Express, Eudora or Netscape Mail, and transports unopened mail from your old inbox.

David Hallowell, a Mozilla contributor, said the main features which make Thunderbird appealing are the user interface and its spam filtering capabilities.

Thunderbird does not include calendar functionality, but this functionality will be available in a separate application, called Sunbird, added Hallowell.

"The main difference between Outlook and Thunderbird is that Thunderbird is simply a mail client and doesn't include scheduling facilities built in -- there are currently volunteers working on the Mozilla Sunbird project which will provide calendaring facilities," he said. "This fits in with the Firefox approach of writing applications to do one task well rather than doing a mediocre job by making an application perform various functions that the user may not require."

Sunbird developers have already released an experimental version of the application, although it is only recommended for testing at present, according to the Sunbird Web site.

The Mozilla Foundation has already experienced considerable successful with its browser, Firefox 1.0, which has been downloaded over nine million times and appears to be taking market share away from Internet Explorer. But Hallowell said he does not believe the foundation plans to promote Thunderbird in the same way.

"I'm unable to speak officially for the foundation on this matter but as far as I know there's no plans for a dedicated Spread Thunderbird site, instead the main aim is to leverage the current Firefox community in promoting Thunderbird," said Hallowell. "Firefox and Thunderbird both share the common goal of making the Internet a safer, less annoying and more enjoyable place so it makes sense to promote Thunderbird as the perfect compliment to Firefox."

Gary Barnett, a research director at Ovum, said that he thinks Thunderbird will do well, but it will take longer for it to make tracks in the market in the way that Firefox has.

"I believe it will be successful but it will be a slower burn than Firefox," said Barnett. "It will undoubtedly be a harder sell."

One of the problems in persuading users to migrate to a new email client is that users tend to make more changes to the settings of their email client, than they do with their browser.

"Getting users to migrate to Firefox is obviously a much higher call -- by using a particular browser you don't sell your soul to it in the same way as you sell your soul to your email client," said Barnett. "Once you've set things up, especially if you have calendaring, it's more effort to move. One of the big show stoppers [for alternative email clients] has been user's reluctance to give up Outlook."

Thunderbird 1.0 can be downloaded from the Mozilla Web site. To read ZDNet's preview review of Thunderbird 1.0 click here.